This research project is to study the effects of plasma inhibitor of profibrinolysin activation. As activator is uded urokinase and streptokinase. Especially will be studied the effect of low molecular weight compounds on the urokinase binding by this plasma inhibitor. Several compounds displace urokinase from this binding and have, therefore, a fibrinolytic effect. We have developed an assay to study the action of this inhibitor undisturbed by antifibrinolysin, the inhibitor of fibrinolysin. We will purify this activation inhibitor and study its properties. The final significance is to acquire fibrinolysis by small amounts of urokinase, infused in the presence of such non-toxic low molecular compounds. Such compounds have a very small fibrinolyric effect even in the absence of urokinase due to liberation of plasmakinase as was demonstrated by von Kaulla. Our methods are aimed to understand the relation between such compounds and the amounts of urokinase of plasma kinase liberated.